The present invention is concerned with the vacuum annealing of worked reactive metal based articles. It is especially concerned with the use of induction heating in vacuum alpha annealing of cold pilgered zirconium base tubing.
Zircaloy-2 and Zircaloy-4 are commercial alloys, whose main usage is in water reactors such as boiling water (BWR), pressurized water (PWR) and heavy water (HWR) nuclear reactors. Those alloys were selected based on their nuclear properties, mechanical properties and high temperature aqueous corrosion resistance.
The history of the development of Zircaloy-2 and 4 is summarized in: Kass "The Development of the Zircaloys" published in ASTM Special Technical Publication No. 368 (1964) pp. 3-27, and Rickover et al. "History of the Development of Zirconium Alloys for use in Nuclear Reactors", NR: D: 1975. Also of interest with respect to Zircaloy development are U.S. Pat. Nos. 2,772,964; 3,097,094 and 3,148,055.
The commercial reactor grade Zircaloy-2 alloy is an alloy of zirconium comprising about 1.2 to 1.7 weight percent tin, about 0.07 to 0.20 weight percent iron, about 0.05 to 0.15 weight percent chromium and about 0.03 to 0.08 weight percent nickel. The commercial reactor grade Zircaloy-4 allow is an alloy of zirconium comprising 1.2 to 1.7 weight percent tin, about 0.18 to 0.24 weight percent iron, and about 0.07 to 0.13 weight percent chromium. Most reactor grade chemistry specifications for Zircaloy-2 and 4 conform essentially with the requirements published in ASTM B350-80 (for alloy UNS No. R60802 and R60804, respectively). In addition to these requirements the oxygen content for these alloys is typically required to be between 900 and 1600 ppm, but more typically is about 1200.+-.200 ppm for fuel cladding applications. Variations of these alloys are also sometimes used. These variations include a low oxygen content alloy where high ductility is needed (e.g. thin strip for grid applications). Zircaloy alloys having small but finite additions of silicon and/or carbon are also commercially utilized.
It has been a common practice to manufacture Zircaloy (i.e. Zircaloy-2 and 4) cladding tubes by a fabrication process involving: hot working an ingot to an intermediate size billet or log; beta solution treating the billet; machining a hollow billet; high temperature alpha extruding the hollow billet to a hollow cylindrical extrusion; and then reducing the extrusion to substantially final size cladding through a number of cold pilger reduction passes (typically 2 to 5 passes with about 50 to about 85% reduction per pass), having an alpha recrystallization anneal prior to each pass. The cold worked, substantially final size cladding is then final alpha annealed. This final anneal may be a stress relief anneal, partial recrystallization anneal or full recrystallization anneal. The type of final anneal provided is selected based on the designer's specifications for the mechanical properties of the fuel cladding. Examples of these processes are described in detail in WAPD-TM-869 dated 11/79 and WAPD-TM-1289 dated 1/81. Some of the characteristics of Zircaloy fuel cladding tubes are described in Rose et al. "Quality Costs of Zircaloy Cladding Tubes" (Nuclear Fuel Performance published by the British Nuclear Energy Society (1973), pp. 78.1-78.4).
In the foregoing conventional methods of tubing fabrication the alpha recrystallization anneals performed between cold pilger passes and the final alpha anneal have been typically performed in large vacuum furnaces in which a large lot of intermediate or final size tubing could be annealed together. Typically the temperatures employed for these batch vacuum anneals of cold pilgered Zircaloy tubing have been as follows: about 450.degree. to about 500.degree. C. for stress relief annealing without significant recrystallization; about 500.degree. C. to about 530.degree. C. for partial recrystallization annealing; and about 530.degree. C. to about 760.degree. C. (however, on occasion alpha, full recrystallization anneals as high as about 790.degree. C. have been performed) for full alpha recrystallization annealing. These temperatures may vary somewhat with the degree of cold work and the exact composition of the Zircaloy being treated. During the foregoing batch vacuum alpha anneals it is typically desired that the entire furnace load be at the selected temperatures for about one to about 4 hours, or more, after which the annealed tubes are vacuum or argon cooled.
The nature of the foregoing batch vacuum alpha anneals creates a problem which has not been adequately addresed by the prior art. This problem relates to the poor heat transfer conditions inherent in these batch vacuum annealing procedures which may cause the outer tubes in a large bundle to reach the selected annealing temperature within about an hour or two, while tubes located in the center of the bundle, after 7 to 10 hours (at a time when the anneal should be complete and cooling begun) have either not reached temperature, are just reaching temperature, or have been at temperature for half an hour or less. These differences in the actual annealing cycle that individual tubes within a lot experience can create a significant variation in the tube-to-tube properties of the resulting fuel cladding tubes. This variability in properties is most significant for tubes receiving a stress relief anneal for a partial recrystallization anneal, and is expected to be reduced by using a full recrystallization anneal. Where the fuel cladding design requires the properties of a stress relieved or partially recrystallized microstructure, a full recrystallization final anneal is not a viable option. In these cases extending the vacuum annealing cycle is one option that has been proposed, but is expensive in that it adds time and energy to an already long heat treatment which may already be taking on the order of 16 hours from the start of heating of the tube load to the completion of cooling.
Additional examples of the conventional Zircaloy tubing fabrication techniques, as well as variations thereon, are described in the following documents: "Properties of Zircaloy-4 Tubing" WAPD-TM-585; Edstrom et al. U.S. Pat. No. 3,487,675; Matinlassi U.S. Pat. No. 4,233,834; Naylor U.S. Pat. No. 4,090,386; Hofvenstam et al. U.S. Pat. No. 3,865,635; Andersson et al. "Beta Quenching of Zircaloy Cladding Tubes in Intermediate or Final Size," Zirconium in the Nuclear Industry: Fifth Conference, ASTM STP754 (1982) pp. 75-95.; McDonald et al. U.S. patent application Ser. No. 571,122 (a continuation of Ser. No. 343,787, filed Jan. 29, 1982 now abandoned and assigned to the Westinghouse Electric Company); Sabol et al. U.S. patent application Ser. No. 571,123 (a continuation of Ser. No. 343,788, filed Jan. 29, 1982, now abandoned and assigned to the Westinghouse Electric Corporation); Armijo et al. U.S. Pat. No. 4,372,817; Rosenbaum et al. U.S. Pat. No. 4,390,497; Vesterlund et al. U.S. Pat. No. 4,450,016; Vesterlund U.S. Pat. No. 4,450,020; and Vesterlund French Patent Application Publication No. 2,509,510 published Jan. 14, 1983.